1. Field of the Invention
This invention relates to a method of measuring internal information from a body being measured (to be referred to as a target hereinafter) by using nuclear magnetic resonance (to be referred to as NMR). More particularly, the invention relates to a method of measured internal information from a target, such as a method of obtaining an image of a cross section of the target placed in a static magnetic field by applying a high-frequency pulse-like magnetic field to a specifically selected small spatial area within the target at a time so as to cause excition of nuclear magnetization therein, detecting NMR generated thereby, and scanning the target by applying a specifying magnetic field to the target for magnetizing a selected area thereof at a time, and the invention aims at improvement of the signal-to-noise (S/N) ratio in the measurement, speeding up of the measurement, simplifying of structure of the magnetic field generator and reduction of electric power consumption.
2. Description of the Prior Art
A number of methods have been proposed heretofore for producing images of the inside of a target, such as a living body, by using internal NMR information of the target.
A typical conventional method of obtaining an image from NMR information comprises steps of exciting magnetization along a line by time-divisional combination of either applications of a high-frequency pulse-like magnetic field so as to selectively excite a specific slice-like area within a target under the presence of a linear magnetic field gradient, said pulse-like magnetic field being shaped so as to have a specific frequency spectrum, or eliminations of magnetization of all other areas of the target except the specific slice-like area, reading out distribution of spin density along said line by using a linear magnetic gradient along the line, and successively scanning the line.
The above-mentioned method of the prior art has a shortcoming in that the method is very complicated to put into practice, because the method uses the linear magnetic field gradient which requires selection of linear on line-shaped areas in the target and, to effect such selection, at least two times of magnetic field applications and switchings of the magnetic field gradient are necessary.
To produce an image from NMR information from the inside of a target such as a living body, several methods have been proposed. However, all of the previously proposed methods use comparatively big NMR magnets, because in order to form a static magnetic field for receiving the entire target cross-section, the NMR magnet is required to have an air gap length which is more than 20 times of that used for usual analysis (which is in general about 30 mm), and this dimensional requirement and weight limitations exclude the possibility of using an iron-core coil for the NMR magnet and only air-core coils have been used therefor.
Besides, it is difficult to produce a very strong magnetic field on a continuous and steady fashion, due to heat generation and other reasons. Even if water cooling is provided, a magnetic field intensity of about 2 kilogauss is an upper limit. Thus, the methods of the prior art have a shortcoming in that a magnetic field generator of fairly large scale is necessary and power consumption therein is fairly large.
In detecting the NMR signals, the problem of S/N ratio is particularly important, and it has been known that the S/N ratio in the NMR signal detection is substantially proportional to the 1.5th power of the intensity of the static magnetic field. Accordingly, from the standpoint of S/N ratio, it is preferable to use as strong a magnetic field as possible. However, in practice, one has to use a comparatively low intensity of magnetic field due to the above-mentioned reasons, and the S/N ratio obtained in each measurement has been comparatively low. Thus, in any of the conventional methods, it has been necessary to make numerous measurements so as to take an average of the thus measured values. Consequently, a considerably long measuring time has been necessary to obtain a reasonably accurate measurement. Another problem which is closely related to the S/N ratio is that of high homogeneity of the magnetic field required for the observation of the NMR signals. Fairly advanced techniques are necessary to achieve a high homogeneity in the case of a strong magnetic field, and this tends to cause a further complication in the device dealing with the NMR signals.